Optical metrology of single features

ABSTRACT

The profile of a single feature formed on a wafer can be determined by obtaining an optical signature of the single feature using a beam of light focused on the single feature. The obtained optical signature can then be compared to a set of simulated optical signatures, where each simulated optical signature corresponds to a hypothetical profile of the single feature and is modeled based on the hypothetical profile.

BACKGROUND

1. Field of the Invention

The present invention relates to wafer metrology, and more particularlyto optical metrology of single features.

2. Related Art

In semiconductor manufacturing, periodic gratings are typically utilizedfor quality assurance. For example, one typical use of such periodicgratings includes fabricating a periodic grating in proximity to asemiconductor chip. By determining the profile of the periodic grating,the quality of the fabrication process utilized to form the periodicgrating, and by extension the semiconductor chip proximate the periodicgrating, can be evaluated.

The profile of a periodic grating can be determined using opticalmetrology. In general, optical metrology involves directing an incidentbeam at the periodic grating, and measuring the resulting diffractionbeam. However, in conventional optical metrology, multiple periods ofthe periodic grating are typically illuminated. Thus, the determinedprofile for the periodic grating is more of an average representation ofthe illuminated periods rather than of an individual period.

SUMMARY

In an exemplary embodiment, the profile of a single feature formed on awafer can be determined by obtaining an optical signature of the singlefeature using a beam of light focused on the single feature. Theobtained optical signature can then be compared to a set of simulatedoptical signatures, where each simulated optical signature correspondsto a hypothetical profile of the single feature and is modeled based onthe hypothetical profile.

DESCRIPTION OF DRAWING FIGURES

The present invention can be best understood by reference to thefollowing description taken in conjunction with the accompanying drawingfigures, in which like parts may be referred to by like numerals:

FIG. 1 depicts an exemplary optical metrology system;

FIG. 2 depicts an exemplary source;

FIG. 3 depicts an exemplary detector;

FIG. 4 depicts another exemplary detector;

FIG. 5 depicts a graph of various exemplary optical signatures;

FIG. 6 depicts an exemplary source and detector;

FIGS. 7-A and 7-B depict a source and detector pair with pupil stops;

FIGS. 8-A and 8-B depict a source and detector pair with pupil stops;

FIG. 9A depicts an exemplary periodic pattern;

FIGS. 9B and 9C depict exemplary diffraction matrices of the exemplarypattern depicted in FIG. 9A;

FIG. 10A depicts an exemplary periodic pattern; and

FIGS. 10B and 10C depict exemplary diffraction matrices of the exemplarypattern depicted in FIG. 10A.

DETAILED DESCRIPTION

The following description sets forth numerous specific configurations,parameters, and the like. It should be recognized, however, that suchdescription is not intended as a limitation on the scope of the presentinvention, but is instead provided as a description of exemplaryembodiments.

With reference to FIG. 1, an optical-metrology system 100 can be used todetermine the profile of periodic grating 102 formed on wafer 104. Asdescribed earlier, periodic grating 102 can be formed in test areas onwafer 104. For example, periodic grating 102 can be formed adjacent to adevice formed on wafer 104. Alternatively, periodic grating 102 can beformed in an area of the device that does not interfere with theoperation of the device or along scribe lines on wafer 104.

As depicted in FIG. 1, optical-metrology system 100 can include anelectromagnetic source 106 and a detector 112. Periodic grating 102 isilluminated by an incident beam 108 from source 106. In the presentexemplary embodiment, incident beam 108 is directed onto periodicgrating 102 at an angle of incidence θ_(i) with respect to normal {rightarrow over (n)} of periodic grating 102. Diffracted beam 110 leaves atan angle of θ_(d) with respect to normal {right arrow over (n)} and isreceived by detector 112.

To determine the profile of periodic grating 102, optical-metrologysystem 100 includes a processing module 114, which converts diffractedbeam 110 received by detector 112 into a diffraction signal (i.e., ameasured-diffraction signal). Processing module 114 then compares themeasured-diffraction signal to simulated-diffraction signals stored in alibrary 116. Each simulated-diffraction signal in library 116 can beassociated with a hypothetical profile. Thus, when a match is madebetween the measured-diffraction signal and one of thesimulated-diffraction signals in library 116, the hypothetical profileassociated with the matching simulated-diffraction signal can bepresumed to represent the actual profile of periodic grating 102.

As described above, in conventional optical metrology, multiple periodsof periodic grating 102 are typically illuminated and thus thedetermined profile for periodic grating 102 is based on an averagerepresentation of the illuminated periods. As described below, in oneexemplary embodiment, optical-metrology system 100 can be used todetermine the profile of a single period of periodic grating 102.Moreover, optical-metrology system 100 can be used to determine theprofile of various types of single features formed on wafer 104, such asa line, space, contact hole, dot, and the like.

More particularly, source 106 can be configured to generate a beam touse in determining the profile of a single feature formed on wafer 104.With reference to FIG. 2, in one exemplary embodiment, source 106 caninclude a light source 202, a collimator 204, and a focusing lens 206.In the present exemplary embodiment, to determine the profile of asingle feature formed on wafer 104, focusing lens 206 is configured tohave a numerical aperture of greater than λ/2d, where λ corresponds tothe wavelength of the light being used and d corresponds to the distancebetween the feature of interest and an adjacent feature. It should benoted that focusing lens 206 can be custom made or adapted from variousexisting types of lenses, such as compact-disc pick-up lens, microscopeobjectives, mononiode optical fiber, and the like.

For example, as described above, the single feature can be a singleperiod of periodic grating 102 (FIG. 1). In this example, d correspondsto the pitch of periodic grating 102 (FIG. 1). For the sake of example,assume that the pitch and thus d is about 500 nm. Also assume for thesake of example that a wavelength of 633 mm is used. As such, focusinglens 206 is configured to have a numerical aperture of greater thanabout 0.6. It should be noted that if the single feature is a line, thend can correspond to the distance between the line and an adjacent line(e.g., the distance between the centers of two adjacent lines).

As depicted in. FIG. 2, source 106 can also include a filter 208.Additionally, source 106 can include an automatic focus control systemand positioning system (not shown) to reduce blurring and center thereference field.

With reference now to FIG. 3, in one exemplary embodiment, detector 112includes a photo-detector 302, a collimator 304, and a focusing lens306. In the present embodiment, diffracted beams are collected anddirected onto photo-detector 302 using collimator 304 and focusing lens306. As noted above, the focusing aperture of the illumination (i.e.,the numerical aperture of focusing lens 206 of FIG. 2) and thecollecting aperture of the detection (i.e., the numerical aperture offocusing lens 306) can be the same or different. Additionally, theaperture shapes can be the same or different.

In the present embodiment, an optical signature can be obtained byscanning the incidence angle of the incoming diffracted beam. Forexample, the incidence angle can be varied through a range by rotatingthe specimen being measured (e.g., wafer 104), moving source 106 (FIG.2) and/or detector 112, or using scanning mirrors.

Alternatively, an optical signature can be obtained by scanning thewavelength of the incoming diffracted beam. For example, the incidentlight can be tuned by a monochromator through a spectral range, or whitelight can be used that is spectrally decomposed in the detection path.

As described below, an optical signature can also be obtained byscanning across the single feature. It should be noted that the opticalsignature can be obtained by one or more combinations of scanning theincidence angle, wavelength of the incoming diffracted beam, and/oracross the single feature.

Additionally, as depicted in FIG. 3, detector 112 can include a filter308 that can generate a weight summation by influencing amplitude aswell as phase of an individual diffracted beam. More particularly, thescattering directions can be weighted and the filter function can beexpressed as A(Θ_(s))exp^(φ(θ)). Thus, in this manner, phase impacts canbe reflected in the intensity signal. Additionally, by adapting filter308 to the type of specimen being used, the sensitivity of themeasurements obtained can be increased.

With reference now to FIG. 4, in another exemplary embodiment, detector112 includes a focusing lens 306 and a detector array 402. It should benoted that the focusing aperture of the illumination (i.e., thenumerical aperture of focusing lens 206 of FIG. 2) and the collectingaperture of the detection (i.e., the numerical aperture of focusing lens306) can be the same or different. Additionally, the aperture shapes canbe the same or different.

In the present embodiment, each cell of detector array 402 can beconfigured to receive information from a certain scattering direction(i.e., angle). An optical signature can then be obtained from (hisangular information. Additionally, spectral information can be obtainedby tuning a monochromatic light source through a wavelength range.Alternatively, spectral information can be obtained by illuminating witha broadband light source and inserting a dispersion element in thedetection path. For example, the dispersion can be performed in asagittal plane. Thus, one coordinate of a 2 dimension detector array 402can be assigned to the scattering angle and the other to the color.

In another exemplary embodiment, an optical signature can be obtainedfor the feature by scanning the focused beam across the feature. Itshould be noted that the optical signature can be obtained solely byscanning across the feature. Alternatively, as noted above withreference to the embodiment of detector 112 in FIG. 3, it should benoted that the optical signature can be obtained by one or morecombinations of scanning the incidence angle, wavelength of the incomingdiffracted beam, and/or across the single feature. With reference to theembodiment of detector 112 in FIG. 4, it should be noted that theoptical signature can be obtained from the angular information andscanning across the feature.

With reference to FIG. 1, the feature can be scanned by moving wafer104, moving source 106 and detector 112, and/or using scanning mirrors.As the feature is scanned, data can be collected at discrete intervals,which corresponds to a sampling rate. Thus, the resolution of theoptical signature obtained can depend, in part, on the sampling rateused.

For example, FIG. 5 depicts optical signatures of diffracted lightscanning across a 0.4 microns wide resist line having a height of 0.7microns formed on a silicon substrate. The optical signatures depictedin FIG. 5 were modeled with a nearly continuous sampling rate. It shouldbe noted, however, that various sampling rates can be used to obtain andmodel the optical signatures. However, as can be seen from FIG. 5, thegreater the sampling rate, the greater the number of data points, andthus the greater the resolution if the optical signatures.

Additionally, the optical signatures depicted in FIG. 5 were modeledassuming a circular illumination and detection aperture. As depicted inFIG. 5, an optical signature was modeled for a line having a rectangularprofile at a numerical aperture (NA) of 0.5 and 0.9. In FIG. 5, for thesake of clarity, the optical signature for a line having a rectangularprofile at a numerical aperture (NA) of 0.5 has been shifted down byabout 5% in normalized reflected intensity. As can also be seen fromFIG. 5, increasing the numerical aperture increases the resolution(i.e., as the slope steepness increases, the image is less blurred).Furthermore, an optical signature was modeled for a line having anotched profile at a numerical aperture (NA) of 0.9. As can be seen fromFIG. 5, the notched profile generates a distinctive optical signature ascompared to the rectangular profile. Thus, optical signatures can beused to determine the profile shape of features.

With reference now to FIG. 6, in still another exemplary embodiment,optical metrology system 100 includes a semi-transparent beam splitter608 to separate the excitation and detection channel of source 602 anddetector 604. In the present embodiment, source 602 and detector 604 usea single focusing lens 606 having a high numerical aperture. Source 602also includes a collimator 610. Source 602 and detector 604 can alsoinclude filters 612 and 614, respectively.

Additionally, detector 604 can include a single photo-detector 302 (FIG.3) or a detector array 402 (FIG. 4). Thus, when a single photo-detector302 (FIG. 3) is used, an optical signature can be obtained by scanningthe incidence angle and/or wavelength of the incoming diffracted beam.When detector array 402 (FIG. 4) is used, an optical signature can beobtained by obtaining the angular information obtained from the cells ofdetector array 402 (FIG. 4). Furthermore, an optical signature can beobtained by scanning the focused beam across the feature.

Additionally, in the present embodiment, one or more pupil stops can beused in the pupil plane to produce oblique incidence. For example, pupilstops can be placed in place of filters 612 and 614 in FIG. 6. Withreference to FIGS. 7-A and 7-B, pupil stops 702 and 708 can bepositioned in place of filters 612 and 614 (FIG. 6), respectively. Pupilstops 702 and 708 include de-centered pupil holes 704 and 710respectively. Thus, in this configuration, the effective numericalaperture (NA_(eff)) is defined by:${NA}_{eff} = {\frac{\mathbb{d}_{s}}{\mathbb{d}_{P}} \cdot {NA}_{P}}$where, NA_(p) is the numerical aperture of the full pupil, d_(p) thepupil diameter, and d_(s) is the diameter of the moving hole in thepupil. As described above, for use in determining the profile of asingle feature, NA_(eff) is greater than λ/2d.

The de-center offset for both pupil holes 704 and 710 can be the same inx and y direction. Additionally, the de-center distance r_(dec) of pupilhole 704 determines the principal angle of incidence (polar andazimuthal). The polar angle of incidence can be determined by:$\theta = {a\quad{\sin\left( {\frac{2r_{dec}}{d_{P}} \cdot {NA}_{P}} \right)}}$

Pupil stops 702 and 708 can then be shifted synchronously to scanthrough the incidence angle. For example, as depicted in FIGS. 7-A and7-B, pupil stops 702 and 708 can be shifted in the direction indicatedby the arrows until pupil holes 704 and 710 reach their normal anglepositions 706 and 712, respectively.

It should be noted that pupil stops 702 and 708 can include variouspupil shapes in addition to simple holes, such as annular, quadropule,and the like. Additionally, the shapes of the illumination stop (i.e.,pupil stop 702) and detection stop (i.e., pupil stop 708) can differ.For example, FIG. 8-A depicts an illumination stop having an annularpupil, and FIG. 8-B depicts a detection stop having a circular pupil.

With reference to FIG. 1, the obtained optical signature (i.e., themeasured optical signature) can be compared to simulated opticalsignatures stored in a library 116. When a match is made between themeasured optical signature and one of the simulated optical signaturesin library 116, the hypothetical profile associated with the matchingsimulated optical signature can be presumed to represent the actualprofile of the feature being examined on wafer 104.

In one exemplary embodiment, the simulated-optical signatures in library116 can be generated using various modal methods, such as rigorouscoupled wave analysis (RCWA), Green Integral Method (GIM), and the like.

For example, efficiencies or complex amplitudes of various diffractionorders, either propagating or evanescent, can be simulated and obtainedusing RCWA. The angular discretization, i.e., the discretization in theβ-space (lateral wave vector component), can be determined by thegrating equation:$\beta_{m} = {\beta_{0} + {m \cdot \frac{\lambda}{d}}}$with β₀ =n sin θ (for classical mount), m=diffraction order, d=distancebetween the feature and an adjacent feature, θ=polar angle of incidence,and λ=wavelength.

These diffraction orders can be referred to as angular- or β-spectrum.Moreover, a modal method can yield a full (complex) diffraction matrixwhen the diffraction matrix is made accessible for further processing.This diffraction matrix can be obtained for both reflection andtransmission, and can couple all outgoing diffraction orders, i.e., theoutgoing β-spectrum to the possible (permitted by the grating equation)incoming directions. In particular, for plane wave excitation, only oneincident direction may be of interest. In this case, only a portion ofthe full information of the diffraction matrix may be used. This featurecan be represented in the following vector-matrix representation:$\begin{matrix}{\left( \overset{\sim}{A} \right)_{o} = {\sum\limits_{i = 1}^{N}{(r)_{o,i} \cdot \left( \overset{\sim}{A} \right)_{i}}}} & (1)\end{matrix}$

Here, (Ã)₀ is the o-th element of a column vector that contains theoutgoing spectrum, (Ã)_(i) is the i-th element of a column vector thatcontains the incoming spectrum and (r)_(o,i) is the o,i-th element ofthe diffraction matrix in reflection. N is the truncation number, i.e.,the total number of diffraction orders involved in the RCWA-computation.For transmission, the matrix r is replaced by the transmission matrix t.

From formula (1), it can be determined that plane wave excitation meansthat there is only one non-zero element in (Ã)_(i), namely the elementassigned to the zero order wave-vector component β₀. This means aprojection of the corresponding column out of the diffraction matrixresults in a column vector (Ã)₀ that contains the complex amplitudes forevery diffraction order for plane wave incidence.

Additionally, in accordance with the concept of angular spectrumpresentation of plane waves in wave optics, every wave-front with knowncomplex amplitude distribution can be decomposed in a spectrum of planewaves. The decomposition procedure is identical with a complex Fouriertransformation: $\begin{matrix}{{\overset{\sim}{A}\left( \overset{\sim}{\beta} \right)} = {\int\limits_{\overset{\rightharpoonup}{r}}{{A\left( \overset{\rightharpoonup}{r} \right)} \cdot {\exp\left( {j\overset{\rightharpoonup}{\beta}\quad\overset{\rightharpoonup}{r}} \right)}}}} & (2)\end{matrix}$

Here, A(r) is the complex amplitude of the wave and r is a positionvector. For numerical reasons the integral is replaced by a sum. Thismeans that the integration boundaries become finite. Actually, thephysical problem is embedded into a finite range, which will be referredto as a super-period P. Due to spatial confinement, the previouscontinuous spectrum turns into a discrete spectrum. Thus, the continuousfunction Ã({right arrow over (β)}) becomes a discrete function that canbe expressed by a vector comprising the elements (Ã)_(m). Applying thisapproach, an arbitrary non-periodic pattern can be treated correctly.

Thus, simulated optical signatures of the diffraction of focused beamcan be generated and obtained as follows:

First, the incident spectrum is computed from the distribution of thecomplex amplitude of a given incident wave by means of formula (2). Inoptical modeling, a Gaussian beam and a circular beam with an Airy-discdiffraction spot are two models that arc widely used as idealized beamshapes for a single mode laser and for a diffraction-limited opticalsystem in connection, with a point source illumination. A Gaussian beamfor example having a waist diameter 2w₀ has the following angularspectrum: $\begin{matrix}{{\overset{\sim}{A}}_{m} = {A_{o} \cdot {\exp\left( {{- \frac{1}{2}}\left( {\frac{2\pi}{\lambda}\beta_{m}w_{o}} \right)^{2}} \right)} \cdot {\exp\left( {j\frac{2\quad\pi}{\lambda}\beta_{m}x_{0}} \right)} \cdot {\exp\left( {{- j}\frac{2\quad\pi}{\lambda}\alpha_{m}z_{0}} \right)}}} & (3)\end{matrix}$where, Ã₀ is the amplitude of the zero-order plane wave component, β_(m)is the lateral wave vector component, and α_(m) is the normal wavevector component of the m-th order. The additional exponential termsexpress an offset of the beam relative to its “zero”-position in lateraldirection (the beam center is offset to the coordinate x₀) and invertical direction (defocus is z₀). An Airy disc (e.g., the intensitydistribution figure in the focus plane of a diffraction limited opticalsystem) entails a simple circ-function as spectrum.

Second, the full diffraction matrix r (or t) is computed by means of arigorous diffraction method, such as RCWA, GIM, and the like.

Third, the diffraction matrix is multiplied with the column vector ofthe incident spectrum resulting in the column vector of the outgoing(diffracted) spectrum.

And next, from the elements of the out-vector, either a total detectoramplitude or intensity can be computed (see equation 4 below and FIG. 3)or the elements can be regarded as direction amplitudes/intensities ofthe scattered beam (FIG. 4).

Additionally, a detector-signal can be obtained by multiplying thevector of the outgoing spectrum by a vector (D)₀ that embodies the(complex) detector function (including of course possible filters, phaseretarders etc.). This yields the complex amplitude A_(d) of theintegrated signal at the detector: $\begin{matrix}{A_{d} = {{\sum\limits_{i = 1}^{N}{(D)_{o} \cdot \left( \overset{\sim}{A} \right)_{o}}} = {\sum\limits_{o = 1}^{N}{(D)_{o} \cdot {\sum\limits_{i = 1}^{N}{(r)_{o,i} \cdot \left( \overset{\sim}{A} \right)_{i}}}}}}} & (4)\end{matrix}$Finally, the intensity is obtained by taking the square:I_(d)∝A_(d)·A_(d)*.

As described above, with reference to FIG. 3, in one exemplaryembodiment, detector 112 includes focusing lens 306 configured tocollect and direct diffracted beams onto photo-detector 302. For thisexemplary embodiment, a maximum numerical aperture value can be obtainedby averaging the intensity over the numerical aperture of focusing lens306 and comparing this value with the plane wave response of theprincipal (i.e., central) “ray” of the focused beam. A normalizeddeviation is obtained. The maximum numerical aperture value can then bedetermined by relating the normalized deviation to an allowed errorlimit.

Additionally, as described above, the diffraction matrix for a periodicpattern can be embedded in a super-period. As depicted in FIGS. 9-A,9-B, and 9-C, a periodic pattern (FIG. 9-A). can cause strong diagonallines that are assigned to certain diffraction orders in the diffractionmatrix (FIGS. 9-B and 9-C). As depicted in FIGS. 10-A, 10-B, and 10-C,at constant wavelength, when the pitch of the periodic pattern increases(FIG. 10-A), the diffraction matrix becomes denser (FIGS. 10-B and10-C). As also described above, the diffraction matrices are excitedwith an input spectrum (i.e., the matrix multiplication of equation 1 isperformed).

As can be seen from FIGS. 9-B, 9-C, 10-B, and 10-C, the resultingoutgoing spectrum excited by a focused incident wave will be affectedonly by the zero-th order (i.e., the main diagonal of the matrices) aslong as the incident spectrum (i.e., the doubled numerical aperture ofthe incident beam) is not wider than the modal distance λ/d. However,conventional optical metrology for use with periodic gratings istypically characterized by the condition: $\begin{matrix}{{2{NA}} = {{2{n \cdot {\sin(u)}}} \leq \frac{\lambda}{d}}} & \left( {5a} \right)\end{matrix}$where u is the aperture angle.

In contrast, as described above, optical metrology for use with singlefeatures can be characterized by the condition: $\begin{matrix}{{2{NA}} = {{2{n \cdot {\sin(u)}}} > \frac{\lambda}{d}}} & \left( {5b} \right)\end{matrix}$When this condition is met, the incident spectrum begins to coverneighboring marginal diagonals. Numerically, this means that theresulting component (or plane wave) of the outgoing wave has to becomputed as shown in equation (1), namely by coherent addition of thecontributions from more than components of the incidence spectrum. Froma physical point of view, this means interference. The optical meaningof high numerical aperture illumination in combination with a low λ/dratio is that a single feature of the pattern can be addressed whileignoring widely the surrounding.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and it should be understoodthat many modifications and variations are possible in light of theabove teaching.

1-55. (canceled)
 56. A system for illuminating a single feature formedon a wafer, the system comprising: a source with a focusing lens tofocus a beam of light onto the single feature, the focusing lens havinga numerical aperture greater than the wavelength of the light useddivided by twice the distance between the single feature and an adjacentfeature; a detector configured to detect beams of light diffracted fromthe single feature; and a processor connected to the detector configuredto determine the profile of the single feature based on the beams oflight detected by the detector.
 57. The system of claim 56, wherein thedetector includes a focusing lens and a single photo-detector.
 58. Thesystem of claim 57, wherein the detector includes a filter configured togenerate a weight summation of the diffracted beams.
 59. The system ofclaim 56, wherein the detector includes a focusing lens and a detectionarray having a plurality of detection cells configured to receivediffracted beams at varying angles.
 60. The system of claim 56, whereinthe source and detector use the same focusing lens, and furthercomprising: a beam splitter disposed adjacent to the focusing lens,wherein the beam of light generated in the source is focused on thesingle feature through the beam splitter and the focusing lens, and thediffracted beams of light are detected in the detector through thefocusing lens and the beam splitter.
 61. The system of claim 60, whereinthe source includes a pupil stop configured to generate obliqueincidence, and the detector includes a pupil stop.
 62. The system ofclaim 61, wherein the pupil stop of the source and the pupil stop of thedetector are configured to synchronously shift from an initial incidenceangle to a final incidence angle.
 63. The system of claim 61, whereinthe pupil stops of the source and the detector have common shapes. 64.The system of claim 61, wherein the pupil stops of the source and thedetector having different shapes.
 65. The system of claim 61, whereinthe detector includes a single photo-detector.
 66. The system of claim61, wherein the detector includes a detection array having a pluralityof detection cells.
 67. A system for illuminating a single featureformed on a wafer, the system comprising: a source with a focusing lensto focus a beam of light onto the single feature, the focusing lenshaving a numerical aperture greater than the wavelength of the lightused divided by twice the distance between the single feature and anadjacent feature; a detector with a focusing lens to focus beams oflight diffracted from the single feature; and a processor connected tothe detector configured to determine the profile of the single featurebased on the beams of light detected by the detector.
 68. The system ofclaim 67, wherein the detector includes a single photo-detector.
 69. Thesystem of claim 68, wherein the detector includes a filter configured togenerate a weight summation of the diffracted beams.
 70. The system ofclaim 67, wherein the detector includes a detection array having aplurality of detection cells configured to receive diffracted beams atvarying angles.
 71. A system for illuminating a single feature formed ona wafer, the system comprising: a source configured to generate a beamof light to be focused onto the single feature; a detector configured todetect beams of light diffracted from the single feature; a singlefocusing lens shared by the source and the detector, the focusing lenshaving a numerical aperture greater than the wavelength of the lightused divided by twice the distance between the single feature and anadjacent feature; a beam splitter disposed adjacent to the singlefocusing lens, wherein the beam of light generated in the source isfocused on the single feature through the beam splitter and the focusinglens, and the diffracted beams of light are detected in the detectorthrough the focusing lens and the beam splitter; and a processorconnected to the detector configured to determine the profile of thesingle feature based on the beams of light detected by the detector. 72.The system of claim 71, wherein the source includes a pupil stopconfigured to generate oblique incidence, and the detector includes apupil stop. claim
 73. The system of claim 72, wherein the pupil stop ofthe source and the pupil stop of the detector are configured tosynchronously shift from an initial incidence angle to a final incidenceangle.
 74. The system of claim 72, wherein the pupil stops of the sourceand the detector have common shapes.
 75. The system of claim 72, whereinthe pupil stops of the source and the detector having different shapes.76. The system of claim 72, wherein the detector includes a singlephoto-detector.
 77. The system of claim 72, wherein the detectorincludes a detection array having a plurality of detection cells.
 78. Amethod of illuminating a single feature formed on a wafer, the methodcomprising: generating a beam of light using a source; focusing thegenerated beam of light using a focusing lens having a numericalaperture greater than the wavelength of the light used divided by twicethe distance between the single feature and an adjacent feature;detecting beams of light diffracted from the single feature using adetector; and determining the profile of the single feature based on thebeams of light detected by the detector using a processor connected tothe detector.